Machines for packaging products with stretchable film include the one described in the Italian patent 1.262.267, belonging to the same applicant, herein incorporated by reference. In these type of machines, the packaging film is supplied by a fixed and horizontal distributor which is comb-shaped and usually placed under horizontal rollers which fold the last edge of the portion of the packaging film under the product previously retained by the distributor, while the same product is translated to the discharge portion of the machine by a pusher installed on the posterior folder. Under this posterior folder a comb-shaped gripper is installed, in an oscillating manner, parallel to the same folder, complementary to the combs of the distributor, which is oriented toward the distributor's comb, with its teeth and by suitable means, can be opened or closed and can be carried from a horizontal, raised position, to a lowered and downwardly inclined position, and vice-versa. When the carriage with the posterior folder performs its approach run to the distributor, the gripper is initially in a raised position and the gripper is closed. Then, the gripper is opened and downwardly inclined in such a manner to fit the teeth of the comb distributor which has left the last edge of the packaging film of the product being discharged and now is supporting the new head of the film which will envelope the following product. At the end of the run of the posterior folder, the gripper closes and grasps the head of the film supported and guided by the distributor. When the posterior folder performs the displacement run from the distributor, the gripper remains closed and initially in a low position to extract the film from the distributor. Subsequently, the gripper is raised horizontally to be pre-arranged for the repetition of a new packaging cycle.
In the gripper according to the prior art, the lower jaw is pushed upwardly by a flat spring which results in difficulty during pre-loading. Difficulties include undesired friction which may lead to possible poor reliability.
In the gripper according to the prior art, in all instances, the opening oscillation movement of the lower jaw, is ensured by the co-operation of a lever keyed on the fulcrum shaft of the jaw, with an oscillating cam pre-arranged on an axle parallel to the working run of the gripper and which is normally maintained in the vertical position by elastic means. When the lever arrives in the final portion of the profile of the cam, the lever comes up against a recess (step) which causes the gripper to close on the film distributor and the end of the same lever is positioned on a laterally inclined portion of the cam, so that when the gripper is moved away from the distributor, the lever causes the cam to oscillate or pivot around its own fulcrum. This action causes remarkable sliding friction between the pin of the lever arm and the cam, makes the active positioning of the same cam unstable and it is not well-suited for the speed and for the high working cycles of the packaging machine.
In all prior devices, the raising and lowering movement of the gripper is ensured by the co-operation of an extension beyond the fulcrum of the stem of the upper jaw of the same gripper, with a little and overhanging cam provided with two consecutive eccentricities, placed with an angular distance between them of about—30–45° and therefore with a steep and short fall between the two eccentricities. This configuration causes concentrated and quick wear of the cam and it is not suitable for the speed and the high working cycles of the packaging machine. Further, the gripper can freely upwardly oscillate.